There are a number of situations where a number of separate channels are combined or multiplexed into a single combined channel. One example of such a situation is in many digital television broadcast systems around the world where the total bit rate of the broadcast channel containing the compressed digital channels is fixed but this value is sufficiently large to allow multiple television channels to be included in the total bit rate. In a satellite direct-to-home system the bit rate of the broadcast channel is defined by the satellite transponder (typically 36 Mbit/s).
The number of bits required to compress many signals varies depending on the complexity of the original signal and the compression scheme used, but typically the compressed bit rate produced from the original signal will not be constant but instead will vary. In order to improve the efficiency of the multiplexing system, the television channels are not coded with a fixed bit rate but rather with a fixed quality where the sum of the instantaneous bit rate of the channels is equal to the fixed total. This is called Statistical Multiplexing or StatMux for short as it is assumed that the channels are statistically independent so when one channel needs more bits to achieve the quality, another will require less. The efficacy of the operation is evaluated on maximising the quality across the channels. The efficacy can be improved by two main methods: encoding efficiency for a single channel; and ensuring that the sum of the instantaneous bit rate does not fall below the fixed total for any period of time.
Increasingly it is desired to be able to transmit additional data together with the multiplexed channels. In the example of the digital satellite system, this additional data might include firmware upgrades for set top boxes (STB), or video-on-demand data that is being trickle-fed to the set top boxes or training course material to be sent to remote sites, or any other data.
This operation represents extra revenue for the broadcaster and is therefore desirable. However, a sizeable part of the broadcast channel may be used for this additional data, so it may have an impact on the quality of the compression coded channels. Previously, a portion of the bit rate of the combined channel is reserved for the additional data. However, typically the bit rate for the data varies significantly over time, and the total bit rate for the broadcast channel will not be fully utilised because of the fixed overhead for the additional data.
The invention seeks to minimise or obviate at least some of the disadvantages of the prior art.